1. Field of the Invention
The present invention relates to a semiconductor micromachine applied to various microsensors and a manufacturing method thereof.
2. Description of the Related Art
Conventionally, there has been developed a micromachining technology that employs semiconductor materials such as silicon. This micromachining technology enables the manufacture of a minute sensor such as an angular velocity sensor (gyro sensor), an acceleration sensor, a microactuator and the like. In combination with a generally employed technology for manufacturing semiconductor circuits or the like, this technology enables the manufacture of the aforementioned minute sensors having a dimension of less than 1 mm, without making use of machining work.
As an example of a product of this technology, a semiconductor micromachine that operates as an angular velocity sensor will now be described with reference to FIGS. 15 through 17.
A semiconductor micromachine 9 as illustrated in FIGS. 15 through 17 has a substrate 92, a movable portion 93 and a pair of stationary portions 97. The movable portion 93 is supported by acicular bodies 95, and is arranged opposite the substrate 92 with a gap 91 provided therebetween. The movable portion 93 is arranged between the stationary portions 97 that are located opposite each other. As shown in FIG. 16, the movable portion 93 is arranged parallel to the substrate 92.
The movable portion 93 is composed of a vibrating plate 96 and movable-side comb-shaped electrodes 961 integrally provided on both sides of the vibrating plate 96.
Each acicular body 95 is connected at an end thereof with a supporting portion 94 secured to the substrate 92. The supporting portions 94 are secured to the substrate 92 through securing layers 949. Each supporting portion 94 is provided with an electrode pad 948.
Furthermore, the substrate 92 has thereon a distance detecting electrode 98, which is located opposite the vibrating plate 96 to detect the distance between the substrate 92 and the vibrating plate 96. As shown in FIG. 16, the vibrating body 96 has on a back surface 962 thereof a detecting electrode section that cooperates with the distance detecting electrode 98.
The distance detecting electrode 98 is connected with an electrode pad 988 through a lead portion 980 and a terminal portion 982.
The stationary portions 97 are provided with stationary-side comb-shaped electrodes 971 for causing the vibrating plate 96 to vibrate. The stationary-side comb-shaped electrodes 971 and the movable-side comb-shaped electrodes 961 are arranged to be engaged with each other. A very narrow gap is formed between each movable-side electrode 961 and each stationary-side electrode 971.
The stationary portions 97 are secured to the substrate 92 by securing layers 979. The stationary portions 97 are provided with electrode pads 978 for applying a voltage to the stationary-side comb-shaped electrodes 971.
In the aforementioned semiconductor micromachine 9, the substrate 92 is made of monocrystal silicon, and the movable portion 93 is made of polycrystalline silicon doped with phosphorus, boron, antimony or the like. The stationary portions 97, the supporting portions 94, and the acicular bodies are also made of polycrystalline silicon doped with phosphorus, boron, antimony or the like.
The distance detecting electrode 98 provided on the substrate 92 is doped with a dopant whose characteristics are different from those of the substrate 92. More specifically, a corresponding portion of the substrate 92 made of p-type monocrystal silicon is doped with phosphorus, boron, antimony or the like, so that the distance detecting electrode 98 is obtained.
The lead portion 980 and the terminal portion 982 are also formed on the substrate 92 substantially in the same manner as the distance detecting electrode 98.
The securing layers 949, 979 are made of a silicon nitridation film.
Furthermore, the electrode pads 948, 978 and 988 are made of conductive materials such as gold, aluminium or the like.
It will be described hereinafter how the aforementioned semiconductor micromachine 9 detects an angular velocity.
First, an alternating-current voltage of a rectangular waveform ranging from 0 to V.sub.0 (V) is applied between the movable-side and stationary-side comb-shaped electrodes 961, 971 on one side. This alternating-current voltage has a resonance frequency for the case where the movable portion 93 resonates in a direction indicated by arrow .alpha. of FIG. 15. An alternating-current voltage having a phase shifted by 180 degrees is applied between the movable-side and stationary-side comb-shaped electrodes 961, 971 on the other side.
There is thus generated an electrostatic force between the respective movable-side and stationary-side electrodes 961, 971. As indicated by arrow .alpha. of FIG. 15, this electrostatic force causes the vibrating plate 96 to vibrate horizontally, that is, in a direction parallel to the substrate 92.
Starting from this state, the semiconductor micromachine 9 is caused to rotate about the c-axis as illustrated in FIG. 15 at an angular velocity .omega..
Then, Corioli's forces F1, F2 as illustrated in FIG. 16 are alternately applied to the vibrating plate 96, which is caused to vibrate in a direction perpendicular to the substrate 92 as indicated by arrow .beta. of FIG. 15.
The Corioli's forces F1, F2 are represented as follows: EQU F1=F2=2m.omega..times.A(2.pi.f)cos{(2.pi.f)t}
wherein "m" represents mass of the vibrating plate 96, ".omega." angular velocity of the semiconductor micromachine 9, "A" amplitude of the vibrating plate 96, "f" frequency of the alternating-current voltage, and "t" elapsed time.
When the vibrating body 96 vibrates vertically, the distance between the vibrating body 96 and the substrate 92, that is, the thickness of the gap 91 changes in accordance with a frequency of the vibration. The change in the distance is detected as a change in the electrostatic capacity between the back surface 962 of the vibrating body 96 and the distance detecting electrode 98. Based on the value thus detected, the angular velocity .omega. is detected by processing signals from a circuit not shown in the drawings.
As will be described hereinafter, the semiconductor micromachine 9 has been conventionally manufactured using a generally employed technology for manufacturing semiconductor circuits.
As shown in FIGS. 18-22, a distance electrode 98 or the like is formed in a substrate 92 by doping the substrate 92 with a dopant. This dopant has a conductivity different from that of the substrate 92.
As shown in FIGS. 18a and 20a, a silicon oxidation film 653 is provided on the substrate 92, an etching stopper layer 654 is then provided on the silicon oxidation film 653, and finally an etching layer 655 is provided on the etching stopper layer 654.
After that, a resist pattern used as a mask is subsequently formed on the etching layer 655 by a photolithographic process. The etching layer 655, the etching stopper layer 654 and the silicon oxidation film 653 are subjected to a RIE (reactive ion etching) process. This etching process allows the formation of contact holes 900 penetrating the silicon oxidation film 653, the etching stopper layer 654 and the etching layer 655 as shown in FIG. 20b (Note that the contact holes 900 can not be seen from a direction shown in FIG. 18b.).
After the resist pattern has been removed, a semiconductor thin film 657 is provided on the etching layer 655 as shown in FIGS. 18c and 20c. By means of ion implantation, the entire surface of the semiconductor thin film 657 is doped with a dopant that has a conductivity equal to that of the distance detecting electrode 98 or the like formed on the substrate 92. The semiconductor thin film 657 is then subjected to a thermal treatment in order to reduce inner stress building up therein and activate the dopant contained therein.
Thereafter, a resist pattern as a mask is formed on the semiconductor thin film 657 by a photolithographic process. The semiconductor thin film 657 is then transformed by an etching process into a movable portion, the acicular bodies and a stationary portion.
A portion of the etching layer 655 located substantially beneath the movable portion and the acicular bodies is removed by an etching process. In the aforementioned etching process, the etchant passes through the introduction holes 679 having a square cross section whose side length is about 4 .mu.m as shown in FIGS. 19a and 21a, reaches the etching layer 655 beneath the semiconductor thin film 657, and erodes the etching layer 655. By the etching process, a gap portion 611 is formed as shown in FIGS. 19b and 21b. In this state, the semiconductor thin film 657 constitutes the movable portion 93, the stationary portion 97 and the acicular bodies.
The movable portion, the stationary portions and the distance detecting electrode 618 are then suitably provided with electrode pads, so that the semiconductor micromachine 9 is completed.
However, the aforementioned semiconductor micromachine 9 has the following drawbacks.
That is, the movable-side comb-shaped electrodes 961 and the vibrating body 96 both belong to the movable portion 93 that is made of a sheet of doped polycrystalline silicon.
Hence, the movable-side comb-shaped electrodes 961 and the vibrating body 96 are electrically in communication with each other.
The movable-side comb-shaped electrodes 961 are spaced apart from the stationary-side electrodes 971 by a very narrow gap. Therefore, when a voltage is applied to the stationary-side comb-shaped electrodes 971, the movable-side comb-shaped electrodes 961 become electrified.
The modulus of elasticity of the acicular bodies 95 connected with the vibrating body 96 needs to be small in order to cause the very light vibrating body 96 to vibrate efficiently. Hence, the acicular bodies 95 generally have a small diameter and a great longitudinal dimension. The acicular bodies 95 thus have an enormous electric resistance value.
Hence, the electric charges accumulated in the movable-side comb-shaped electrodes 961 are unlikely to move towards the acicular bodies 95 whose electric resistance is great. Instead, the electric charges are accumulated in the vibrating plate 96.
Consequently, the back surface 962 of the vibrating body 96 is charged with an excessive amount of electric charges, so that the detected distance between the back surface 962 and the distance detecting electrode 98 is not exactly proportionate to the distance between the vibrating body 96 and the substrate 92.
That is, the angular velocity .omega. cannot be detected precisely.
In other words, in the conventional semiconductor micromachine, electric charges are likely to move between the electrodes and wires, which causes crosstalk of signals.
As described hitherto, the circuit constituting the electrodes and the wires of the semiconductor micromachine has a low S/N ratio, so that the detecting precision thereof deteriorates. Furthermore, since a plurality of electrodes within the movable portion are electrically in communication with each other, the circuit should be designed such that these electrodes are provided with an equal electric potential (these electrodes are grounded). Accordingly, the circuit has a low degree of design freedom.
In the case where the micromachining technology is applied to the acceleration sensor, the microactuator or the like, the same drawbacks as in the angular velocity sensor will be observed.